Electron tube structure



July 31, 1951 J. 2,562,820

ELECTRON TUBE STRUCTURE Filed March 15, 1950 s Sheets-Sheet 2 l a w 1 587 l k i g K36 f2? l -12 1 l .95 i J f M21 I .19 L EH2 V I 7)] 58 59 i ll L1 I A h A I l kl l 3! 1 J -7JL I a 7.9 77 a 75 75 a! a? iooooaoooeoga a I l y 31, 1951 J. D. REID 2,562,820

ELECTRON TUBE STRUCTURE I Filed March 15, 1950 3 Sheets-Sheet 5 IINVENTOR. i I 7 JOHN 0. REID L Q i 822% Q W ATTORNE X Patented July 31,1951 ELECTRON TUBEIsTRUoTURE I John Drysdale Reid, Cincinnati, Ohio,assignor' to Avco Manufacturing Corporation, Cincinnati, Ohio, acorporation of Delaware 1 Application March 15, 1950, Serial No. 149,836

22 Claims. (Cl. 81558) The present invention relates to a vacuum tubeand circuit particularly suitable for operation in; the very highfrequency and ultra high frequency ranges.

In designing and operating circuits in the very .1 'high frequency (VHF)and ultra high frequency (UHF) ranges, it has been found that sucheffects as cathode inductance, grid inductance, input capacitance andcircuit connection inductances are sufilciently great so as to produceundesirable or detrimental eifects and various techniques directed tothe solution of problems presented by these factors have been developedin the prior art. Tubes have been scaled down to smaller size in orderto reduce lead and electrode length, with a resulting decrease inelectrode inductance, and capacitance, as well as lead inductance. Also,it has been suggested that leads of relatively large diametershould beemployed to reduce lead inductance and skin effect losses.

In view of the increasing commercial importance of the VHF and UHFranges, it would be desirable to further decrease the loading effect ofelectrode and lead inductance, thereby increasing the useful frequencyrange beyond which a resonance effect with inter-electrode capacitanceresults. It furthermore would be desirable to provide an arrangementwherein the major portion of the circuit is included in the sameenvelope as a vacuum tube. By providing built-in coupling capacitors itis possible to produce vacuum tubes and circuits having known constantsso that reliable operation may be obtained in the UHF ranges.

The present invention minimizes lead loss by including inter-electrodeconnections in the tube envelope. Electrode inductance is reduced to aminimum in accordance with its teachings by means of a plurality ofsymmetrical connections to the electrode structures, whereby the signalsource effectively sees only one-fourth of the inductive reactance whichis seen when conventional connections to the electrode structure areused.

It is, therefore, an object of the present invention to provide animproved vacuum tube construction particularly suited for operation atthe higher frequencies.

A further object of the present invention is to provide an improvedvacuum tube construction having built-in coupling capacitors.

Another object of theinvention is to provide an improved vacuum tubeconstruction wherein the envelope contain the elements of the thermionicthe circuit in which the thermionic tube elements are to operate. 7

A still further. object is to provide an improved vacuum tubeconstruction wherein symmetrical electrode connections are utilized toreduceinherent iru'luctance and.capacitance loading effects of the tubeelectrodes and connecting leads.

Other and further objects of the present invention subsequently willbecome apparent by reference to the accompanying drawings, wherein:

Fig. 1 is a diagrammatic view of a tube construction incorporating theprinciples of the present invention;

Fig. 2 is a base layout showing the symmetrical arrangement desired; a

Fig. 3 is a circuit diagram which is the equivalent of the constructionillustrated in Fig. 1;

Figs. 4, 5 and 6 illustrate mixer or converter application circuitsembodying the principles of the present invention;

Fig. 7 is a diagrammatic representation illustratingan oscillator tubeconstruction in accordance withv the invention;

Fig. 8 is a diagrammatic representation of a pentode tube constructionin accordance with the invention;v

Fig. 9 illustrates a typical base for a vacuum tube constructed inaccordance with the Fig. 8

embodiment of the present invention;

Fig. 10 .shows a suitable envelope; Fig. 11 is a simplified end view ofthe tube structure shown in Fig. 1;

Fig. 12 is an equivalent circuit diagram of the construction illustratedin Fig. 7; and

Fig. 13 shows a simplified schematic diagram .which is the equivalent ofthe construction shown in Figs. 7 and 12.

Fig. 1, along with end view, Fig. 11, illustrates the construction of atriode suitable for operation as a mixer or converter. The tube has anindirectly heated cathode l2 provided with a connection at each end soas to provide terminals 13 and M. A suitable filament l5 receivescurspaced relation to the grid structure I8 there is an anode l9.

Two connections 2| and 22 are provided for the anode l9.

The ends ofthe grid l8 are connected to inductors 23 and 24 which areconnected to capacitor sleeves 25 and 26, respectively. The capacitor,sleeves 25 and 26 constitute relatively short cylinders surrounding aportion of the indirectly heatedcathode l2. Each of the cylinders.nection. cathode inductance is also, lowered proportion- .ately bysimilar construction.

The degenerative effect on the input signal is 25 and 26 is separatedfrom direct contact with the cathode |2 by a suitable insulating sleeve(such as the air gaps shown) which preferably has such qualities as toprovide a high resistance leakage path. Thus, a grid leak resistor isprovided in parallel with a capacitor formed by each of the cylinders 25and 26, the leakage of the air dielectric being equivalent to theparameters 20, 30 in Fig. 3.

Surroundin the anode I9 is another cylinder 21, which is electricallyconnected to the cathode l2, thereby to produce a capacitor 21 betweenthe anode I9 and the cathode I2.

Each end of the cathode I2 is connected to an inductor. Thus, one end ofthe cathode I2 is connected to an inductor 28, which has its other endconnected to a terminal 29. The inductor 28 is inductively coupled tothe inductor 23, which is connected to the grid I8. At the other end ofthe tube structure an inductor 3| is connected to the cathode I2 and toa terminal 32. The inductor 3| is inductively coupled to the inductor24, which is connected to the grid l8.

In order to understand the principles involved in the tube constructionas described above, some of the effects that the leads and electrodes ofa conventional tube have on a (UHF) signal must be considered. Theconventional cathode and cathode return lead inductances carry both theplate current and any grid-cathode current flowing from the input signalsource. Therefore, the voltage drop caused by anode current flowingthrough the cathode inductance has a degenerative efiect, on the inputsignal.

some proportion related to the frequency of the applied current. Also,it is well known that an increase in lead length increases the inductivereactance which the lead offers to UHF currents. Therefore, it can beseen, when the grid cathode circuit of a conventional tube is analyzedwith a view to using the tube in UHF work, that the physical lengths ofthe grid structure and the cathode structure are important factors to beconsidered. In conventional tube constructions, a lead from a base pinis connected to one end of the grid. Also, a similar lead is connectedto one end of the cathode structure. This means that the full physicallengths of the grid structure and cathode structure, as well as thelengths of their respective leads, must be considered in determining theinherent inductive reactance of the tubes internal grid-cathode path.The present invention effectively lowers the inductance ,oifered by thetube electrodes, by bringing in leads to both ends of the grid and toboth ends of the cathode. Since the circuit is symmetrical,

each input lead to the grid carries one half of the signal currentrealized in the grid cathode circuit and the inherent inductance of thegrid wire is effectively broken into two parallel paths. Each path hasan inductance equal to one half of the inductance of the whole grid andsince the paths are effectively connected in parallel, the inductivereactance of the grid is reduced to 25% of the inductance of aconventionally connected grid. In other words, a conventionalconnection, i. e. to one end of the grid, results in a grid inductancefour times as large as that produced by the above described double endcon- It can now readily be seen that the further reduced by taking theanode return current through two paths i. e. the two leads connected toterminals l3 and I4, directly to both ends of the cathode structure I2,thus reducing to a minimum the cathode circuit inductance common to thegrid and plate circuits.

It will become apparent to those skilled in the art that the structurerepresented in Fig. 1 may be enclosed within a relatively compactenvelope and that for the purpose of symmetry and minimum inductiveeffects, the leads from the various inductors and tube elementspreferably are arranged as illustrated in Fig. 2.

Fig. 3 shows the equivalent circuit diagram wherein it will be recalledthat imperfect insulation provides a grid leak 20 in parallel with acapacitor 25 and a grid leak 30 in parallel with a capacitor 26. Theinductive reactance of the control circuit input leads is also greatlyreduced by the construction shown in Fig. 1.

As is stated above, the inductance of a lead increases as its length isincreased. In order to keep these leads as short as possible, therebykeeping their inductive reactance as low as possible, inductors 23 and24 are connected directly between the grid l8 and the capacitancesleeves 25 and 25, inside of the envelope. The illustrated symmetricalconstruction places these inductors effectively in parallel, therebymaking their combined inductance equal to one half of the inductance ofa single path. The dual connections to the cathode reduce the cathodestructure in- 28 and 3| is connected to the cathode which is connectedthrough an inductor 34 to ground. The inductor 34 is coupled to anotherinductor 35 which is connected to a suitable source of oscillationswhich are to be mixed with the incoming energy from the transmissionline 33. Since the grid circuit inductors 23 and 24 are in parallel, thetotal inductance connected between the grid and capacitors 25-28 is onlyhalf as much as the inductance of each single inductor and this has beenrepresented as a single inductance in Fig. 4. Likewise, the capacitors25 and 28 and the accompanying grid resistors may be represented by asingle capacitor and grid resistor.

Fig. 5 illustrates the manner in which the structure illustrated inFigs. 1, 2 and 3 may be employed with an unbalanced input wherein one ofthe conductors of the transmission line 33 is connected to bothinductors 28 and 3|, and the other conductor of the transmission line 33is connected to the common juncture of the inductors 28 and 3|. Asuitable tuning capacitor 36 may be connected across the line 33 toprovide for variable tuning of the input circuit. The anode IQ of thevacuum tube is coupled through a .capacitor 31 to a suitable source ofoscillations.

The anode I9 is connected through the primary winding 38 of anintermediate frequency transformer to a suitable source of anodepotential 5. suit of this internaI connection the RF and'osciL- latorfrequencies have a low impedance cathode return path which is relativelyindependent of the impedance of the IF output leads connected betweenplate I9 and terminals 2| and 22.

Fig. 6 shows still another arrangement wherein the anode l9 is connecteddirectly to the anode 4| of a triode vacuum tube having a grid 42 and acathode 43. The anodes l9 and 4| of the two vacuum tubes are connectedtogether and through the primary winding 38 of the first intermediatefrequency transformer to the terminal 39, which is connected to thesource of anode voltage. The grid 42 is connected to ground through gridcapacitor 44 wich is shunted by a grid resistor 45. The groimded sidesof resistor 45 and capacitor 44 are also connected to one end of aninductance 46 having an intermediate point connected to the cathode 43.The other end of the inductor 46 is connected through a capacitor 98 tothe anode 4|.

Referring to Fig. '7 a vacuum tube construction and circuit isillustrated, which is particularly adapted for oscillator use. Thecathode is indirectly heated by a suitable filament having electricalterminals 52 and 53. Each end of the cathode 5! is also provided withterminals 54 and 55. At one end of the cathode 5| there is provided aninductor 56 connected thereto, which is also connected to an externalterminal 51. At the other end of the cathode 5| there is a similarinductor 58 connected to the cathode and to an external connection orterminal 59.

Inductor 56 is coupled to an inductor 6| connected between the gridstructure 62 and cylinder 63. The cylinder 63 surrounds the anode 64 soas to provide a capacitor between the grid 62 and the anode 64. Gridleak resistor 99 is connected between the cylinder 63 and the cathode5|, thereby furnishing a D. C. path between the said grid and cathode.The anode 64 receives potential through a resistor 65 connected to aterminal 66. At the other end of the grid 62 there is another inductor61 which is also connected to the cylinder 63. It will be noted thateach end of the anode 64 as at 68 and 69, is of smaller diameter so asto be closer to the cathode 5| and the inductors 6| and 61. Thisproduces an increased capacitive eifect at these points. The structureshown in Fig. 7 is one form of a Colpitts oscillator, as can readily beseen by reference to the simplified circuit diagrams of Figs. 12 and 13.The increased capacitive effect realized by decreasing the end diameterof anode 64 at 68 and 69, in Fig. 7, is diagrammatically shown bycapacitors 68C and 69C in Fig. 12. Capacitors (390, Fig. 12, includesthe inherent grid cathode capacitance between the grid and cathodestructures.

In Fig. 13 the circuit of Fig. '7 and Fig. 12 has been furthersimplified into a more easily recognizable Colpitts circuit. CapacitorCsc illustrates the shield cathode capacitance, while capacitor Cgcillustrates the grid cathode capacitance.

Since the Colpitts circuit is conventional and well known, an exhaustiveexplanation is not required. However, briefly referring to Fig. 13, itcan be seen that coils til-61 and capacitors Cs: and Cgc form a resonantcircuit. The voltage across the resonant circuit is divided into twoparts by the series circuit comprising condensers Gel: and Cgc. Thecathode 5| is effectively coupled to the common connection between thesecapacitors. Therefore, the instantaneous voltages at the two ends of theresonant circuit are #5 6'. opposite ln'polarity with respect to thecathode and in the right phase relation to sustain oscillations.

Fig. 8 shows the principles of the present invention, applied to apentode amplifier. An indirectly heated cathode H is provided withconnections at each end 12 and 13. The cathode H is heated by a filamentenergized through connections l4 and 15. The cathode is surrounded by agrid 16 which is connected at each end thereof to an inductor 11 and aninductor I8. Inductor I1 is connected to a capacitor sleeve 19surrounding a :portion of the cathode H. In a similar manner, the otherinductor 18 is connected to a capacitor sleeve 8|, surrounding a portionof cathode H. Capacitor sleeve 8| is connected through blocking resistorI82 to terminal I08, thereby adapting the grid of the amplifier forconnection to a D. C. grid bias source. The inductor 18 is effectivelycoupled to another inductor 82, connected between the cathode II and theterminal 83. In a similar manner, the inductor 11' is coupled to aninductor 84 connected between the cathode l I and a terminal 85.

Just beyond the grid 16 is a screen grid 86 which is connected to a,terminal 81 through a blocking resistor ltll. One end of the screen grid86 is connected to a capacitor coupling sleeve 88, whereas the other endof the screen grid 86 is-connected to a capacitor coupling sleeve 89.Thus, a certain by-pass capacitance exists between the screen grid 86and the cathode 1|.

Just beyond the screen grid 86, there is a suppressor grid 9|, which ateach end is connected to the cathode II. Beyond the suppressor grid 9|there is located an anode 92, which is connected to a terminal 93. It isto be noted that a D. C. path is not provided through the dielecover thecathode surface directly beneath the control grid structure. The ends ofthe cathode,

which act as a condenser plate for condensers 25 and 26 in Fig. land forcondensers 19, 8| 88 and 89 in Fig.8, are uncoated.

The various terminals for the pentode, vacuum tube and circuitillustrated in Fig. 8 are shown in Fig. 9. The elements of the vacuumtube and circuits may be enclosed in an envelope generally having theappearance illustrated in Fig. 10.

It is to be noted that the longitudinal axes of the internal tubestructure is parallel to the base plane surface in lieu of beingperpendicular thereto as in a conventional tube. This constructionallows the leads extending to the exterior to be of equal length.

From the foregoing embodiments it will be appreciated that theprinciples of the present invention ofier numerous advantages overarrangements in the prior art. In the mixer or converter application, asshown in Figs. 6 and 5, the oscillator voltage is supplied by couplingor directly connectin the oscillator source to the plate circuit. Thecapacity existing between the grid and plate, transfers this energy tothe grid circuit. It will be appreciated that where, as

in the structure indicated, the input capacitance is low, a largevoltage will appear on the grid. Furthermore, if the mutual conductanceis high,

anyplate variations will have no substantial detrimental effect.

"In constructing such an arrangement as that suggested by the embodimentdescribed in connection with Fig. l, the capacitances provided by thecapacitors 25 and 26 would each be about ten times the usual inputcapacitance of the tube structure. It will be recalled that thedielectric sleeves which separate the cylinders 25 and 26 from thecathode, were selected so as to be imperfect insulators so that gridleak values of about one megohm could be obtained. The capacitanceprovided by the capacitor 21 should be large enough to provide acomparatively low impedance at signal frequencies. This capacitancewould have a value approximately equal to ten times the capacitanceexisting between the grid and the plate. It will be recalled that thecylinder 21 which forms one plate of the capacitance also serves as ashield for the plate 19, thus to reduce any oscillator radiation.

While for the purpose of describing and illustrating the drawings,certain embodiments have been specifically shown in the drawings and itis to be understood that the invention is not to be limited thereby,since such variations and other embodiments are contemplated as may becommensurate with the spirit and scope of the invention as defined bythe accompanying claims.

I claim:

1. A vacuum tube for a symmetrical circuit comprising a cathode, aconcentric grid and a concentric anode, symmetrically arranged inductiveelements located between each end of said grid and said cathode, andsymmetrically arranged inductive elements located adjacent said firstinductive elements and provided with connections extending to theexterior of said vacuum tube.

2. The combination comprising an evacuated envelope containing anindirectly heated cathode, concentric grid and anode structures,symmetrically arranged inductive elements located between each end ofsaid grid and of said cathode and connected in circuit with said tubeelements,

symmetrically arranged inductive elements located adjacent said firstinductive elements provided with connections extending to the exteriorof said envelope and symmetrically arranged connections extending fromsaid cathode, said grid and said anode to the exterior of said envelope.

3. The combination comprising an evacuated envelope containing anindirectly heated cathode, concentric grid and anode structures, aplurality of capacitors each comprising an insulated metal sleevesurrounding said cathode, symmetrically arranged inductive elementslocated at each end of said grid and connected between said grid andsaid capacitors and symmetrically arranged inductive elements locatedadjacent said first inductive elements and provided with connectionsextending to the exterior of said envelope.

4. The combination comprising an evacuated envelope containing anindirectly heated cathode, concentric grid and anode structures, aplurality of capacitors each comprising an insulated conductive sleevesurrounding said cathode, symmetrically arranged inductive elementslocated at each end of said grid and connected between said grid andsaid capacitors and symmetrically arranged inductive elements locatedadjacent first inductive elements, each of said elements being connectedbetween said cathode and the exterior of said envelope.

5. The combination comprising an evacuated cluding an indirectly heatedcathode, having con- 8 4 centric grid and anode structures,symmetrically arranged inductive elements located at each end of saidgrid and capacitively coupled to one of said thermionic tube elementsand symmetrically arranged inductive elements coupled to said firstelement and connected between said cathode and the exterior of saidenvelope.

6. A converter and circuit comprising an evacuated envelope containingthermionic tube elements including an indirectly heated cathode having aconcentric grid and anode, symmetrically arranged inductive elementslocated at each end of said grid, a plurality of cylindrical sleevessurrounding said cathode adjacent each end and separated therefrom by ahigh resistance dielec tric material, said inductive elements beingconnected between said grid and said cylindrical sleeves andsymmetrically arranged inductive elements coupled to said first elementsand connected between said cathode and the exterior of said envelope.

7. The combination comprising an evacuated envelope containingthermionic tube elements including an indirectly heated cathode havingconcentric grid and anode structures, a concentric sleeve surroundingsaid anode, a plurality of inductances each connected between an end ofsaid grid and said sleeve, a plurality of inductances coupled to saidfirst inductances and connected between said cathode and the exterior ofsaid envelope, said anode having end structures of smaller diameter toprovide capacitive coupling between said anode and said cathode andsymmetrically arranged connections from said cathode to the exterior ofsaid envelope.

8. The combination comprising an evacuated envelope containingthermionic tube elements including an indirectly heated cathode havingconcentric grid and anode structures, a plurality of metal sleevessymmetrically arranged adjacent the ends of said cathode and insulatedtherefrom, a plurality of inductances each connected between an end ofsaid grid and one of said sleeves, a plurality of inductance coupled tosaid first inductances and connected between said cathode and theexterior of said envelope and symmetrically arranged connections fromsaid cathode to the exterior of said envelope.

9. The combination comprising an evacuated envelope containingthermionic tube elements including an indirectly heated cathode providedwith concentric grid and anode structures, one of said grid structuresbeing a screen grid, a plurality of metal sleeves symmetrically arrangedadjacent the ends of said cathode and insulated therefrom, a pluralityof inductances each connected between an end of the control grid and oneof said sleeves and connections between said screen grid and certain ofsaid sleeves, a plurality of inductances coupled to said firstinductances and connected between said cathode and the exterior of saidenvelope and symmetrically arranged connections from said cathode to theexterior of said envelope.

10. The combination comprising an evacuated envelope containingthermionic tube elements including an indirectly heated coated cathodehaving concentric control. screen and suppressor grid structures, aconcentric anode therefor, a plurality of metal sleeves symmetricallyarranged adjacent the uncoated ends of said cathode and insulatedtherefrom, said screen grid bein connected at its ends to certain ofsaid sleeves, said suppressor grid being connected at its ends to 76symmetrical points on said cathode, a plurality of inductances connectedbetween each end of the control grid and one of said sleeves, aplurality of inductances coupled to said first inductances and connectedbetween said cathode and the exterior of said envelope and symmetricallyarranged connections from said cathode to the exterior of said envelope.

11. An electron tube comprising a control electrode and a cathodeelectrode input circuit, an output circuit including an anode, separatesymmetrical built-in connections to both ends of one of said inputcircuit electrodes, an envelope for containing said electrodes and saidbuilt-in connections and signal supplying means coupled to said built-inconnections for supplying an input signal between both of saidsymmetrical built-in connections and the other input circuit electrode,whereby said symmetrical built-in connections afford substantiallyidentical impedance paths from each end of said one electrode to oneside of said signal supplying means.

12. An electron tube comprising a control electrode and a cathodeelectrode input circuit, an output circuit including an anode, firstseparate symmetrical built-in connections to both ends of said controlelectrode, second symmetrical builtin connections to both ends of saidcathode electrode, an envelope for containing said electrodes and saidbuilt-in connections and signal supplying means coupled between saidfirst and second built-in connections whereby said symmetrical built-inconnections afiord substantially identical impedance paths from each endof said control electrode to one side of said signal supplying means andalso aiford substantially identical impedance paths from each end ofsaid cathode electrode to the other side of said signal supplying means.

13. In a vacuum tube structure the combination comprising an anode, acontrol grid and a cathode sleeve covered with an electron emissivecoating having an uncoated end portion, a builtin capacitor comprising aconcentric jacket spaced from said portion by a dielectric and a highlyevacuated envelope for containing said anode, cathode, control grid andbuilt-in capacitor.

14. An electron tube comprising an internal tube structure contained inan evacuated envelope which includes a plurality of tubular shapedelectrodes in concentric spaced relationship, connections between atleast two of the said tubular electrodes and the exterior of theenvelope, at least one helical grid structure between two of saidelectrodes and in concentric spaced relation therewith, a first twoterminal inductive means having one terminal connected to one end ofsaid grid structure, a second two terminal inductive means having oneterminal connected to the other end of said grid structure, means forcoupling the second terminals of the first and second inductive means toopposite ends of one of said plurality of tubular shaped electrodes, athird inductive means located adjacent and inductively coupled to saidfirst said inductive means and a fourth inductive means located adjacentand inductively coupled to said second inductive means, said third andfourth inductive means being provided with connections extending to theexterior of said vacuum tube.

15. An electron tube structure as defined in claim 14 wherein thelongitudinal axes of the said tubular shaped electrodes and helical gridstructure is parallel to the plane surface of the tube base.

16. An electron tube structure as defined in claim 15 wherein the saidmeans for coupling the second terminals of the first and secondinductive means to opposite ends of one of said plurality of tubularshaped electrodes includes a pair of tubular sleeves each in concentricspaced relationship with an uncoated end portion of a partially coatedcathode and separated therefrom by a high resistance dielectric.

17. An electron tube structure as defined in claim 15 wherein the saidmeans for coupling the second terminal of the first and second inductivemeans to opposite ends of one of said plurality of tubular shapedelectrodes comprises symmetrical connections to each end of an outercapacitor sleeve surrounding the anode structure.

18. An electron tube internal structure as defined in claim 15 whereinthe said connections between at least two of the said tubular electrodesand the exterior of said envelope comprise connections to the cathodeand anode electrodes.

19. An electron tube internal structure as defined in claim 18 whereinthe said connections to the cathode comprise two symmetrical leads eachconnected to opposite ends of the said cathode structure.

20. An internal electron tube structure as defined in claim 18 whereinthe said connections to the anode comprise two symmetrical leads eachconnected to opposite ends of the said anode structure.

21. In a vacuum tube structure of the type which includes at least ananode, a control grid and a cathode, a built-in capacitor comprising acathode element having an uncoated end portion and a concentric jacketspaced from said portion by a dielectric having a high resistanceleakage path, whereby there is both an A. C. path and a D. C. pathbetween said cathode sleeve and said concentric jacket.

22. In a vacuum tube structure of the type which includes at least ananode, a control grid and a cathode, the combination comprising acathode structure, a concentric anode structure spaced from said cathodeand an encircling concentric capacitor sleeve spaced from said anode andhaving a plurality of symmetrical connections between the ends of saidcathode and the ends of said capacitor sleeve.

JOHN DRYSDALE REID.

REFERENCES CITED The following references are of record in the

